In the field of feeding, separating and sorting documents, there have been many different mechanisms and devices designed to feed and to convey or transport such documents along a path wherein it is important that each and every document being transported follows in successive manner and that two or more documents do not overlap during the time of transport when processing the documents.
Known mechanisms and devices have included mechanical apparatus such as a lever or like member which would be positioned in one operable condition when there was a single document passing a particular station and such lever would be in another operable condition when two or more documents were passing at the same time. Another known apparatus took the form of a light source and a light-sensitive device which would have an output at one level when a single document was being fed and an output at another level when the documents were in overlapped condition. Still other apparatus made use of air pressure as well as the absence of air altogether to detect overlap feeding of documents.
Representative prior art in the area of detection of double documents includes U.S. Pat. No. 2,994,528, issued to H. E. Hull et al on Aug. 1, 1961, which discloses a device for detecting the feeding of overlapping documents and wherein ports for detecting documents are formed in two opposed spaced walls of a channel and the ports are connected with a vacuum source. When two documents are fed in overlapping relation into the channel, the lower atmospheric pressure inwardly of both ports and the atmospheric pressure between the documents causes the documents to cover the ports which causes a pressure drop indicating the presence of overlapping documents.
U.S. Pat. No. 3,026,419, issued to J. I. Aweida et al on Mar. 20, 1962, discloses an overlap detector using photoelectric cells to discover an overlap condition and wherein the change in intensity of a beam of light alters the output of a solar cell with the output being adjusted as a function of the thickness of an average document.
U.S. Pat. No. 3,182,301, issued to E. R. Kolb on May 4, 1965, discloses a multiple sheet detector with sensing means responsive to the change in thickness of the stream of sheets and includes control means for performing a control operation when the change in stream thickness is more than that of a single sheet thickness and with subsequent pulses having magnitudes dependent upon the sheet thickness and analyzed both as to magnitude and time to determine whether or not the sheets are properly passing a particular station.
U.S. Pat. No. 3,283,163, issued to A. J. Folmar on Nov. 1, 1966, discloses a photosensitive overlapped document detector utilizing a light source and a light-sensitive element along with apparatus for separating overlapped documents and wherein light from the source passes through one document and then is attenuated and thereafter the attenuated light is of insufficient strength to operate the light-sensitive element in the same manner as would a single document and a signal is developed for indicating overlapped documents.
U.S. Pat. No. 3,627,303, issued to A. Zeewy on Dec. 14, 1971, discloses a circuit for providing a signal for incorrect thickness of a book and wherein the signal operates with a delay to prevent operation of the machine and thereby reject the book. The delay for rejection is introduced by a register which is shifted by a clock pulse derived from a switch.
And, U.S. Pat. No. 3,731,916, issued to R. S. Pettet on May 8, 1973, discloses discriminating apparatus for moving translucent sheets and utilizing a photoelectric detection device in a circuit along with a first differential amplifier to produce a first signal output when the device detects light in excess of a first predetermined value and a second differential amplifier adapted to produce a second output signal when the device detects light in excess of a second predetermined value.